The IMS (Internet Protocol Multimedia Subsystem) enables an operator, i.e. a service provider, to deliver multimedia services based on IP (Internet Protocol) transport in a secure way, independently of the access technology and of the type of user terminal. A conventional IMS architecture includes three main layers, typically referred to as an access layer, an IMS layer (or a session setup/control layer), and a service layer. The core network, which comprises an HSS (Home Subscriber Server) and a CSCF (Call Session Control Function), can be connected to several different access networks, both cellular and wire-lined, and the PSTN (Public Switched Telephony Network) is normally accessibly via a MGW (Media Gateway) and a MGCF (Media Gateway Control Function).
The SIP (Session Initiation Protocol) is the main signalling protocol for handling a packet switched session in the IMS, and the CSCF processes the SIP signalling and provides session control and interaction with the HSS. The functions of the CSCF can be divided into a Serving Call Session Control, S-CSCF, an Interrogating Call Session Control, I-CSCF and a Proxy Call Session Control, P-CSCF.
The session- and the user management take place in the IMS core network, which comprises the HSS and the CSCFs, of which the CSCFs are managing the sessions between users and services, and the HSS contains subscription-related information and location information and performs authentication and authorization of the users. Further, a border gateway of the IMS network typically acts as a SIP firewall, allowing only secure flows. This border gateway node may be denoted e.g. a Session Border Gateway, SBG, a Session Border Controller, SBC, or a Border Gateway function. Hereinafter, the generic term “border gateway” will be used, and the abbreviation SBG (Session Border Gateway) will be used in the drawings.
The specification “SIPconnect 1.1 Technical Recommendation” specifies a profile of the Session Initiation Protocol, SIP, for handling the connectivity between a SIP-enabled Service Provider Network and a SIP-enabled Enterprise Network. Said SIP Connect-specification describes two modes of operation, i.e. the Registration mode and the Static mode, which differ in the way the Service Provider Network discovers the SIP signalling address of a telephone exchange, i.e. a private branch exchange serving a particular company/enterprise. Such a private branch exchange may be denoted e.g. a PBX (Private Branch Exchange) or a PABX (Private Automated Branch Exchange). Hereinafter, the generic term “private branch exchange” or the abbreviated term PBX will be used. Further, the telephone lines between the operator and a private branch exchange are typically denoted trunk lines, or simply a PBX trunk.
In the Registration mode of the SIP Connect, also referred to as the Dynamic node, a private branch exchange, PBX, registers with the service provider's network using a Register requesting message including a specifically formatted Contact URI, followed by an authentication procedure using the SIP Digest. When the PBX is authenticated by the service provider's network, a DNS (Domain Name System) is dynamically updated with each SIP signalling address associated with the PBX, thereby connecting the PBX trunk to the network of the service provider.
In the Static mode of the SIP Connect, the service provider's network is either pre-configured with the (static) signalling addresses associated with the private branch exchange, or obtains the addresses from a DNS-server.
Conventionally, a PBX is connected to an IMS network of a service provider using e.g. the ISDN (Integrated Services Digital Network). However, since the ISDN may not be available in the near future, it is desirable that the PBX can connect to an IMS network using the above-described standardized SIP Connect, instead of the ISDN. However, at present this is not possible, since the IMS does not support SIP Connect, and consequently a service provider is not able to connect a PBX trunk to an IMS network using the SIP Connect.